1. Field of Invention
The present invention relates to a transparent and spread illuminating apparatus which is formed in a unit with a display unit used in a front-surface illuminating means for a variety of reflection type display units and so on, and more particularly to a transparent and spread illuminating apparatus used as a front-surface illuminating means in a reflection type liquid-crystal display unit.
2. Background of Related Art
A liquid-crystal display unit operable with a low electric power consumption has been increasingly in demand for display units mainly applied to computers because it is thin in configuration, light in weight and so on. Since liquid crystal which is a structural element of the liquid-crystal display unit emits no light by itself, an illuminating means for illuminating an image is required, which is different from a light emission type device such as a CRT. In particular, in the circumstances where a demand of fine and colored image in high level is increased recently, a structure in which a liquid-crystal display unit is backed with a high luminance spread light source is usually used. However, since, in order to illuminate the spread light source, an excessive electric power is required, there occurs such a problem that the feature of liquid crystal having a low electric power consumption is lessened.
In particular, a portable liquid-crystal device frequently used with the advantages of the liquid-crystal display unit thin in configuration and light in weight has a drawback that the consumption of an internal electric power becomes increased due to the illumination of the spread light source which is provided in the liquid-crystal display unit to remarkably shorten a period of illuminating time during for using it as the portable device.
In order to solve the above problem, there has been developed a reflection type liquid-crystal device that may operate by employing an ambient light as an illuminating means even if no spread light source is provided.
The most basic structure of the reflection type liquid-crystal device is designed in such a manner that two flat glass substrates, on each of one surfaces of which a transparent electrode is disposed and on each of the other surfaces of which a polarization plate is disposed, are formed so as to oppose their transparent electrodes to each other at a given interval, and liquid crystal material is filled between the respective glass substrates. Furthermore, a color filter is disposed on the glass substrate which is on an observation face side, and a high-efficient reflector is disposed on the glass substrate which is on a back surface side.
In this example, the transparent electrode plate of the glass substrate which is on the back surface side is patterned, and in order to display a desired image, switching devices are connected to the transparent electrode plate, respectively.
In the reflection type liquid-crystal device thus structured, since an ambient light incident to the reflector disposed on the back surface is reflected so as to illuminate a screen, an image on the screen can be observed.
However, since the reflection type liquid-crystal device has a structure such that, as mentioned above, an ambient light incident to the reflector illuminates the screen, its display quality depends on the ambient brightness. In particular, in demand for high quality concerning image on display, since a constitution of the color display reflection type liquid crystal, the demand of which is expected to be increased, has to become such that the color filter and the like are added thereto, its reflectivity becomes lower than that of monochrome liquid crystal. Accordingly, in a state where the amount of light to be irradiated on the screen is a little (that is, the surrounding is relatively dark), because the luminance of the screen is not sufficient, an auxiliary illumination is required for observing an image.
As an appropriate auxiliary illuminating means for the above reflection type liquid-crystal device, there has been disclosed a transparent and spread illuminating apparatus in Japanese Patent Application No. Hei 9-347648.
A transparent and spread illuminating apparatus 1' shown in FIG. 11 is disposed so as to cover the observation face F of the above-structured reflection type liquid-crystal device L for use, and its structure is such that a linear light source lamp 4 is disposed so as to be close to one side end surface 3 of the flat transparent substrate 2 which is made of a material high in transmittance and shaped in a rectangle in section as shown in FIGS. 11 and 12. As the light source lamp 4, a cold cathode fluorescent tube (CCFL), a heat cathode fluorescent tube (HCFL) or the like is used.
In this example, it is assumed that, in FIG. 12, one surface (a lower side in FIG. 12) of the transparent substrate 2 which abuts on the reflection type liquid-crystal device L is a lower surface 5, and its opposite surface (an upper side in FIG. 12) which is on an observation face (screen) side is a top surface 6.
On the top surface 6 of the transparent substrate 2 is formed a light reflection pattern 7. The light reflection pattern 7 is made up of a large number of grooves 8 which are substantially triangular in section and a large number of flat portions 9 adjacent to the grooves 8.
The light reflection pattern 7 is designed in such a manner that intervals between which the grooves 8 are defined are different depending upon the position of the groove so that the brightness becomes nearly uniform at any positions in the transparent substrate 2 without being influenced by the distances from the light source lamp 4 as shown in FIG. 12. In other words, the ratio of the width (occupied area) of the grooves 8 to the width (occupied area) of the flat portions 9 is set so as to gradually increase as the grooves or the flat portions become farther from the one side end surface 3 of the transparent substrate 2.
With the addition of the transparent and spread illuminating apparatus 1' thus structured as an auxiliary illumination, a light emitted from the light source lamp 4 is made incident to the interior of the transparent substrate 2 from the one side end surface 3 of the transparent substrate 2, and progresses toward the opposite surface 10 while the light repeats reflection and refraction in the interior of the transparent substrate 2. During this action, the light is emitted from the lower surface 5 of the transparent substrate 2 little by little with the result that the light is irradiated on the reflection type liquid-crystal device L which is disposed in close contact with the transparent substrate 2. Moreover, since the light reflection pattern 7 is formed on the transparent substrate 2, the distribution of the light emitted from the lower surface 5 can become substantially uniform entirely thereon.
Although being omitted from showing in FIGS. 11 and 12, since an outer surface of the light source lamp 4 which is not faced on the one side end surface 3 is covered with a film-shaped reflection member, the coupling efficiency of a light can be enhanced. Furthermore, when the side surfaces of the transparent substrate 2 except for the one side end surface 3 are also covered with a reflection member, since the light is prevented from being emitted from the side end surfaces, the amount of light emitted from the lower surface 5 of the transparent substrate 2 can be increased. In particular, on the opposite surface 10 of the one side end surface 3, since the amount of emitted light is larger than those of two other side surfaces, it is desirable that the opposite surface 10 is covered with a reflection member.
Also, since a direction of the light emitted from the lower surface 5 of the transparent substrate 2 varies by changing an angle of reflection of the light in accordance with the configuration of the grooves 8 of the light reflection pattern 7, the configuration of the groove 8 can be appropriately set so that a large amount of light is emitted in a direction perpendicular to the lower surface 5 (that is, a front-surface direction).
As described above, in order to ensure the luminance of the screen of the reflection type liquid-crystal device L, the transparent and spread illuminating apparatus 1' serving as the auxiliary illumination is added to the observation face F side so that the screen can be observed without being influenced by the ambient brightness.
However, it has been found that the addition of the transparent and spread illuminating apparatus 1' may lead to a case where a moire pattern occurs on the observation face F of the reflection type liquid-crystal device L.
The moire pattern is an interference stripe which is caused, when a light is reflected on the light reflection pattern 7 of the transparent substrate 2, by occulting stripes caused by different transmission rate between the grooves 8 and the flat portions 9, and a mosaic-pattern arrangement (a dotted line indicated in the reflection type liquid-crystal device L in FIG. 11) of liquid-crystal cells which form pixels of the reflection type liquid-crystal device L. This moire pattern causes such a problem that an image on screen is difficult to be observed.
As described above, it has been proved that the occurrence of the moire pattern is closely related to the pattern configuration of the light reflection pattern 7, that is, the depth, size and pitch P (a distance from one of the grooves 8 to another groove 8 adjacent thereto), etc., of the grooves 8. Accordingly, as a result of studying the pattern configuration of the light reflection pattern 7 by being variously changed, there has been proved that in the case where the pitches P of the grooves 8 (that is, the widths of the flat portions 9) are not constant as shown in the light reflection pattern 7 in FIG. 12, the large moire pattern occurs locally, thereby to make it difficult to observe the image on screen.